A conventional motorized wheelchair is typically equipped with a chassis having front wheels that consist of a pair of free-spinning castors and rear wheels that consist of a pair of motor-operated wheels which are fixed to the chassis and are frequently driven independently of one another by reversible, variable-speed DC motors. In such wheelchairs, the rotative direction and speed of each of the fight and left motor-operated wheels are varied by reversing the power source connection of the appropriate DC drive motor and by regulating the input voltages to the motor, which actions result respectively in switching of the direction of movement of the wheelchair (between forward and backward travel) and changing of the speed (rpm) of movement of the wheelchair on or along an underlying ground surface. In this manner, a conventional motorized wheelchair is fairly easily advanced, retreated, turned to the right or to the left, and turned around in a stopped state.
However, such a conventional motorized wheelchair cannot be moved laterally with the rider remaining face forward since the motor-operated wheels cannot be pivoted so that they are directed sideways with respect to the rider. This restriction in movement interferes with the rider's ability to utilize the wheelchair completely, to move with total freedom of motion, and to perform certain functions that would typically be available only through lateral motion as, for example, painting on a horizontally-elongated surface or writing on a blackboard. Moreover, currently known drivewheel control arrangements further restrict the range of wheelchair movements in additional ways and may thereby limit a user's flexibility of motion along particular paths or directions of travel.
Traditional steering assemblies for wheelchairs are intrinsically restricted in their freedom of motion as a consequence of their design. Such assemblies typically provide a centralized steering device having transmission members attached to the drivewheels for organized and controlled cooperative motion between the wheels. For example, U.S. Pat. No. 4,852,679 to Fry discloses a series of four wheels pivotally movable about vertically-oriented axes. The four wheels are not, however, each independently so pivotable but, rather, are tied together by chains for commonly-steered pivotally-rotative positioning. In other words the positioning is achieved by a central, motorized transmission device causing the wheels to move in-tandem, not independently.
Another example of known arrangements is disclosed in U.S. Pat. No. 4,513,832 to Engman, in which a chassis incorporates three pairs of wheels. Each pair of wheels of the two outer (that is front and rear) sets are freewheelingly, independently pivotable about substantially vertical axes, while the wheels of the central or interior wheel sets are nonswivelably attached to the chassis structure and are motor driven for effecting driven movement of the wheelchair along a support surface.
Nora, et al. (U.S. Pat. No. 4,483,805) addresses the problem of lateral motion by disclosing rear drivewheels that are independently driven by separate electric motors and are pivotally movable about substantially vertical axes. However, as in the other prior art drivewheels, pivotable movement is effected in-tandem, in that simultaneous adjustments are made to the relative rotative positions of the wheels as the wheelchair is selectively steered. In addition, each of the rear drivewheels is limited in the extent to which it is pivotable about a respective kingpin that attaches it to the frame, since each drivewheel is operated in-tandem with the other and armatures protrude from each drivewheel assembly thereby preventing the drivewheel from pivoting in a complete circle. Thus, although the Noda, et al. device permits pivoting of the drivewheels for achieving lateral motion, the device does not allow the wheels to be independently turned through a 360.degree. pivot about a substantially vertical axis wherein the kingpin (that attaches the wheels to the chassis) resides. This limitation results in a decrease in the overall performance and movability of the chassis; since the orientation of the wheels of this device cannot move independently, but only in-tandem, they cannot be placed in different lateral orientations relative to the chassis and the effective turning radius of the wheelchair is thus restricted by the finite variety of motions made possible by the in-tandem pivoting of the drive wheels.